DC--DC static converters are widely used in power supplies, actuating systems, displays, signal processing systems, and the like. Static DC--DC converters are based upon well known basic circuit topologies or configurations, often used alternatively to one another, as for example the so called "flyback" topology, wherein the primary winding of a transformer is driven by a power switch, controlled by a PWM system, switching at a frequency established, for example, by a sawtooth local oscillator. The oscillator has a natural frequency which may be fixed upon connecting an external RC network to the integrated circuit of the controller. The switching frequency may be modified with respect to its design nominal value or otherwise set by a system's clock signal or any equivalent synchronizing signal. In the case of a so-called "boost" topology it is an energy storing inductor that is similarly driven.
As is well known, static converters may function either in a so-called discontinuous current mode (DCM) or in a continuous current mode (CCM). In the former (DCM) mode each cycle begins with the current in the inductor or primary winding of the transformer starting from zero, because the inductance has been allowed to discharge completely during the preceding phase. In contrast, in a CCM mode, the current through the primary winding or the inductor at the beginning of each cycle starts from a value different from zero. The DCM mode is characterized by a regime of triangular shaped currents flowing through the power switch, while the CCM mode is characterized by trapezoidal shaped currents flowing through the power switch.
It is important that the respective current-mode control circuits be effective in controlling the output current level so to prevent damage in case of overloads. A generally followed approach includes using the signal present on a dedicated sensing resistor connected in series with the power switch to activate a current limiting loop when a certain voltage threshold is exceeded on the sensing resistor.
This type of control is commonly referred to as being of the peak current type. It is also known that in these peak current control systems, the output power of the converter increases as the switching frequency increases. In a DCM mode, the increase of the power that may be output is substantially proportional to the switching frequency, while in a CCM mode such an increase of the output power, as a function of the switching frequency, is less marked (nonlinear relationship).
Controllers have been marketed for a long time as they provide an integral part of the majority of current mode PWM system devices and frequently they also include the power switching device. These integrated devices are, for obvious reasons multipurpose. In other words, they are designed for use in different applications, allowing the implementation of supplies and converters of different topology by simply connecting the device in an external circuit which includes components connectable to the respective pins. Often, these integrated devices are produced within a standard package, such as, for example, the DIP16 and SO16W, both having 16 pins.
Among these commercial devices are the L4990 manufactured by SGS-THOMSON MICROELECTRONICS; UC3842A/B manufactured by SGS-THOMSON MICROELECTRONICS, TEXAS INSTRUMENTS, MOTOROLA, SAMSUNG, UNITRODE et al.; LT1241 manufactured by LINEAR TECHNOLOGY; UC3801 and UC3828 manufactured by UNITRODE. Datasheets of all these devices include their functional schemes and characteristics and are readily available. A typical commercial product is the L4990 manufactured by SGS-THOMSON MICROELECTRONICS, a high level block diagram of which is shown in FIG. 1.
In many applications, the switching frequency may vary within a wide range of frequencies that may be either programmable or synchronized by means of clock signals or pulses that may be generated by deflecting circuits, as in the case of CRT monitors. Often the dependency of the output power from the switching frequency may not be tolerated especially in applications destined to function in a wide range of switching frequencies.
In these particularly critical applications use has been made of dedicated circuits capable of implementing an output power compensation so to prevent an excessive increase in output power upon an increase of the switching frequency. For example, in a CRT monitor the synchronizing pulses derived from the deflection circuit are integrated by an R-C lowpass filter to generate a DC voltage proportional to the frequency. This DC signal is applied to the pin of the integrated controller which senses the current crossing the transformers primary winding of a flyback converter. In this way, the controller is deliberately "cheated" to "sense" a current that is in fact higher than the real one as the frequency increases. In this way the maximum current limit is reached with a lower peak current.
Nevertheless, these known approaches have the following drawbacks:
a) the amplitude and duration of the synchronizing pulses must be known and constant and the compensation circuit must be necessarily trimmed according to these parameters;
b) the compensation circuit is not integratable in the controller's functional circuit because of the need to realize a compensation circuit ad hoc, bearing in mind the characteristics of the specific application;
c) the correction effected by the circuit is essentially linear, while the required correction may not be so, ideally; and
d) the design and realization of the compensation circuit implies a non-negligeable cost.